1. Field of the Invention
The invention pertains to a fill-level indicator for indicating the level of the contents of a container or the storage level of a material at a storage place, whereby said indicator comprises an ultrasonic transducer that is surrounded by a housing and operated to act alternately as a pulse emitter and as a receiver for the pulses reflected on the surface of the material and an electric circuit for exciting the transducer.
2. Description of Prior Art
Fill-level indicators of this type are frequently utilized for monitoring different processes, e.g., detecting the fill-level of containers or storage places or ascertaining similar measuring values required for carrying out said processes. Such indicators are used for determining the fill-level, i.e., the contents of a container or the storage level at a storage place, from the time that elapses while the pulse travels from the membrane of the ultrasonic transducer to the surface of the material that acts as the plane of reflection and back to the transducer.
In order to generate the original pulse, the ultrasonic transducer that is surrounded by a housing comprises a piezoelectric crystal that is excited to oscillate at the frequency of the ultrasonic wave via the original pulse generator formed by the electric circuit. According to the state of the art, the face surface of the piezoelectric crystal that faces the membrane of the transducer and a short section of the outer surface of the piezoelectric crystal is covered by a plastic layer that serves for adapting the acoustic impedance of the ceramic material of the crystal to the acoustic impedance of the medium into which the acoustic pulse is emitted.
At the side situated opposite the membrane, a damping layer consisting of plastic fills out the remaining part of the inner space of the housing. This plastic surrounds the other face surface as well as the largest portion of the outer surface of the piezoelectric crystal. The damping layer is provided so as to influence the oscillating behavior of the crystal in such a way that the oscillating direction is essentially oriented toward the membrane and the emission of oscillation energy is damped in any other direction.
According to German Patent Application No. P 4,230,773.2, this damping layer may be composed of a silicone elastomer with a high content of metal oxides. Such a damping layer influences the oscillating behavior of the crystal and prevents the undesired emission of oscillation energy in the radial direction as well as the axial direction extending away from the membrane.
With sensors in which the inner housing is filled with a plastic layer, this may, however, not be realized entirely due to the promotion of the sound propagation by the plastic layer, so that a residual quantity of acoustic energy is still emitted primarily in the radial direction from the outer surface of the crystal and consequently not available as acoustic energy that is directed toward the material. If using the fill-level indicator for measuring the level of the contents in a container, this radially emitted acoustic energy is reflected on the container wall, on installations, or on the inner walls of sound conveyance pipes and consequently directed into the measuring space which, in turn, leads to the fact that the measurement is influenced by interfering secondary echoes.
Increasing the wall thickness of the plastic layer enclosing the outer surface of the piezoelectric crystal such that the sound propagation path is elongated and the sound absorption is increased is usually done in an attempt to eliminate this disadvantage. The arrangement of a layer that consists of a cellular material and surrounds the piezoelectric crystal between the crystal and the plastic layer is intended to attain the same objective.
On the other hand, the total diameter of the housing of such an indicator should be as small as possible so as to allow the opening in the container, through which the indicator is inserted into the housing or the diameter of a sound conveyance pipe, to be designed correspondingly small.
One electroacoustic transducer of the same type is disclosed in German Patent No. 4,027,949. This patent proposes damping the undesired sound emission extending parallel to the axial direction of the acoustic head by means of a filler ring that surrounds the acoustic head, while forming an intermediate space, that consists of a plastic material that is reinforced with glass fibers. However, the aforementioned publication does not propose any measures for reducing or preventing sound waves that are emitted in the radial direction.